Every year in the U.S., 180,000 new cases of breast cancer are diagnosed and approximately 60% of these are estrogen-receptor-positive (ER+) (Martin et al. 1994). Moreover, every year there are a substantial number of breast cancer recurrences, and many of these are ER+. Tamoxifen has been the mainstay for medical treatment of ER+ breast cancer and has provided significant clinical benefit (Fisher et al. 1989; Fisher et al. 1998). However, there are a substantial number of ER+ breast cancers that are refractory to tamoxifen due to either intrinsic or acquired resistance. New treatments for these ER+, tamoxifen-refractory breast cancers are needed, and some promising agents are currently being evaluated in clinical trial. Letrozole, which blocks estrogen synthesis by inhibiting aromatase; and goserelin, which stifles ovarian release of estrogen by inhibiting gonadotropin release; are both being tested for this purpose (Goss et al. 2001; Nystedt et al. 2000).
Several population and epidemiologic studies as well as laboratory studies have indicated that alpha-fetoprotein (AFP) interferes with estrogen-dependent responses, including the growth-promoting effects of estrogen on breast cancer (Bennett et al. 1998). For example, Couinaud et al. (1973) have reported that women with AFP-secreting hepatomas develop amenorrhea which self-corrects following removal of the hepatoma, and Mizejewski et al. (1983) have shown that AFP inhibits the responsiveness of the uterus to estrogen. Jacobson et al. (1989) and Richardson et al. (1998) have shown that elevated levels of AFP during pregnancy are associated with subsequent reduction in lifetime risk for breast cancer, and Jacobson et al. have hypothesized that this could be caused by a diminution in estrogen-dependent breast cancers (Jacobson et al. 1989). Sonnenschein et al. (1980) have shown in rats that an AFP-secreting hepatoma prevents the growth of an estrogen-dependent breast cancer in the same rat. Finally, it has been shown that AFP purified from a human hepatoma culture and then injected into tumor-bearing immune-deficient mice stopped the growth of ER+, but not estrogen-receptor-negative (ER−) human breast cancer xenografts in these mice, and did so by a mechanism different from that of tamoxifen (Bennett et al. 1998).
More recently, the active site of AFP responsible for its antiestrotrophic activity has been identified (Mesfin et al. 2000). It consists of amino acids 472–479 (SEQ ID NO: 6, EMTPVNPG), an 8-mer sequence in the 580-amino acid AFP molecule.
Aggregation of proteins and peptides has been seen with full length AFP as well as with subunits of AFP. Wu et al. (1985) showed that AFP tends to form aggregates, which may contribute to its loss of anti-estrotrophic activity during storage. Eisele et al. (2001) reported that oligomers of various sizes formed during storage of a 34-mer peptide (amino acids 447–480) derived from AFP. Similar aggregation behavior has been seen with a number of other protein and peptide pharmaceuticals, including human interferon-γ (Kendrick et al. 1998), human calcitonin (Bauer et al. 1994), insulin (Sluzky et al. 1991), and synthetic β-amyloid peptide (Hilbich et al. 1991; Christmanson et al. 1993; Halverson et al. 1990). Hughes et al. (1996) and Hilbich et al. (1992) reported inhibition of amyloid peptide aggregation by substitution of hydrophobic phenylalanine with hydrophilic threonine or by adding polylysine at the carboxy-terminus of the amyloid peptide.